He could be feeling a sense of calm. Naruto has a certain presence that even in sleep can be soothing. Sasuke, who has had a tumultuous life, might find peace in just observing Naruto at rest.
Well, Boruto might feel a sense of pride seeing his father Naruto in action against Sasuke. He has heard so many stories about their battles, and now seeing it in a fanfiction version could make him realize the deep bond and rivalry between them. He might also analyze their fighting styles and try to incorporate some of their moves into his own combat techniques later on.
Well, it could be a complex mix of shock, anger, and betrayal. Shock because it's unexpected, anger towards the person doing the seducing, and betrayal as it involves one's spouse.
He might feel anger. Since he was so set on revenge against Itachi, any portrayal that didn't match his view of himself as the victor could make him angry.
The character might be really intrigued. They could be thinking about how the fanfiction is changing or expanding the original Undertale story. Maybe they are comparing it to their own experiences in the Undertale world, if they are a character from that universe.
Love, definitely. Sora's sacrifice for Kairi is likely to be driven by deep love. Also, there could be sadness as the characters are separated in such a tragic way.
Maybe they embark on an adventure to help a small, oppressed village. They use their combined skills to overthrow the tyrant ruling the village and then stay for a while to help rebuild it. During this time, they learn about different ways of life and make new friends. This adventure would strengthen their bond and also their reputations as great shinobi.
One possible reason could be his deep concern for Naruto. Despite their complex relationship, Sasuke might be worried about Naruto's well - being after all they've been through. He could be silently guarding him while he sleeps, a sign of his unspoken affection and protectiveness.
He might be really surprised. He could find it strange to see different versions of his own story and the people around him in the fanfiction.
She might be thinking about how the fanfiction either stays true to the original relationship dynamics between her and Naruto or how it changes in interesting ways.
He might be really amazed. He'd see different versions of his world and himself, which could make him think about his own life and choices in a whole new light.